1. Field of the Invention
The present invention relates to an apparatus for determining the authenticity that determines an authenticity of a sheet to which is attached such a substance that excites a light including a second wavelength longer than a first wavelength when irradiated with a light including the first wavelength.
2. Description of the Related Art
There has been known such an apparatus for determining the authenticity that conveys a paper sheet such as a gift certificate with a conveying mechanism, and determines whether the paper sheet is authentic or counterfeit with an optical sensor capable of emitting/receiving a light. The light can be a visible light or an infrared light. Furthermore, from the viewpoint of prevention from counterfeiting the paper sheet, characters or a pattern has been printed on the paper sheet with phosphor-containing ink, which is invisible to the human eye, in recent years.
When irradiated with an excitation light of a predetermined wavelength (for example, a visible light or an infrared light), such phosphor-containing ink emits an invisible light (for example, an infrared light) of a different wavelength from that of the excitation light.
There has been developed an apparatus for determining the authenticity that makes use of these characteristics of the phosphor-containing ink. Specifically, a latent-image mark is printed in advance on a paper sheet with anti-counterfeit ink such as phosphor-containing ink. The apparatus for determining the authenticity emits an excitation light of a predetermined wavelength to the paper sheet, and detects the latent-image mark by receiving a fluorescence emitted from the phosphor-containing ink. The apparatus for determining the authenticity determines the authenticity of the paper sheet based on whether the latent-image mark is detected.
For example, Japanese Patent Application Laid-open No. 2007-136838 discloses such a technology that an invisible bar code formed with anti-counterfeit ink that emits an infrared light when irradiated with a visible light or an infrared light is printed in advance on a printed matter. Therefore, the authenticity of the printed matter can be determined by reading the bar code.
However, when such authenticity determination of a paper sheet with the use of characteristics of anti-counterfeit ink, which emits a light of a different wavelength from that of an excitation light, is performed, an overlap between the wavelength of the excitation light and the wavelength of the light emitted from the anti-counterfeit ink becomes a problem. Specifically, a light source (for example, a light-emitting diode) for emitting an excitation light to a paper sheet emits a light in a predetermined range of wavelengths. That is, the excitation light may include a light of the same wavelength as the light emitted from the anti-counterfeit ink.
In other words, a light-receiving sensor receives the excitation light, which is reflected on the paper sheet or passes through the paper sheet, together with the light emitted from the anti-counterfeit ink. Therefore, it is difficult for the light-receiving sensor to receive only the light emitted from the anti-counterfeit ink. Consequently, in the past, a detection accuracy of a latent-image mark printed with anti-counterfeit ink or the like was not good enough, so that an accuracy of authenticity determination of a paper sheet was insufficient as well.
Incidentally, as a light source for emitting an excitation light, the one capable of emitting a light of a single wavelength can be used. However, such a light source is expensive, so that the use of the light source will disadvantageously lead to the increase in production cost of the apparatus for determining the authenticity.